1. Field of the Invention
The present invention relates to a photomask fabrication method for a semiconductor device, and, more particularly, to an improved phase shifting photomask fabrication method for fabricating a rim type and an out-rigger type photomask.
2. Description of the Related Art
A known lithographic process employs a photomask having a transparent portion for passing light and an opaque portion for blocking light in order to form a pattern on a wafer by irradiating the light onto a ready-made imprint pattern. Accordingly, a variety of such photomasks exhibiting a light phase difference have been disclosed in recent years.
An SPIE concept of a phase shifting photomask was initially disclosed by M. D. Levenson of IBM in 1982, and a variety of so called "Levenson" phase shifting photomasks have been described since.
A phase shifting photomask shifts by 180.degree. the phase of light passing through a light shifter relative to light passing through a transparent portion (other than the light shifter), in order to improve resolution and focus depth of a photolithographic apparatus. Such a phase shifting photomask may be an alternating type, an attenuating type, a rim type, an out-rigger type, or the like.
A conventional phase shifting photomask of the rim type and the out-rigger type will now be described.
Referring to a rim type phase shifting photomask as shown in FIG. 1A, an opaque layer 11 and a light shifter 12 are formed on a transparent photomask substrate 10. Inside the light shifter 12 there is an opening pattern 13, which is not imprinted by the light.
FIG. 1B, which is a composite view, shows a cross-sectional view taken along line IB--IB in FIG. 1A, a graph illustrating light intensity when the light that passed through the light shifter 12 and the opening pattern 13 impinges on a wafer 20, and a cross-sectional view of a patterned photoresist film 18 formed on a wafer 20.
The light passing through the photomask substrate 10 goes through the opening pattern 13 and is phase-shifted while passing through the light shifter 12. No light passes through the opaque layer 11. The intensity of the light that passed through the opening pattern 13 is much higher than the intensity of light that passed through the light shifter 12, which is a narrow strip.
When the light reaches the photoresist film 18 formed on the wafer 20, a portion of the photoresist film 18 below the opening pattern 13 is exposed and may be removed for a positive photoresist, forming a corresponding opening 23 on the wafer 20.
As shown in FIG. 2A, the transparent photomask substrate 10 has a pattern that includes an opaque layer 11, a pair of phase shifters 12, and a pair of opening patterns 13. The edges of the pair of phase shifters 12 are abutted or adjacent to each other. Here, because the pair of phase shifters 12 have light phases identical to each other, the light intensity on a portion of the photoresist layer 18 between openings 23 is higher due to a light combination effect. The intensity of the light that passed through the photomask substrate 10 is shown in FIG. 2B. The light with a relatively high intensity passes through the marginal portion of each of the abutted phase shifters 12, so that the intensity of the light that passed through the photomask 10 produces a graphic image as shown in FIG. 2B. The photoresist film 18 just below the opening pattern 13 is exposed and may be removed entirely, forming the openings 23. At the same time, the photoresist film 18 between the openings 23 is partially removed, forming a depression 22, which is disadvantageously formed in a central portion of the photoresist film 18 between the openings 23.
In order to overcome such a disadvantage, a phase shifting photomask for shifting a light phase between the opening patterns 13 in the mask is disclosed in U.S. Pat. No. 5,302,477, a plan view of which is shown in FIG. 3A.
A pair of opening patterns 33 and 36 serving as a transmissive region are formed on a transparent quartz substrate 20. A pair of light shifters 32 and 35 are formed surrounding the opening patterns 33 and 36. A chromium layer 21 serving as an opaque layer is formed on the transparent substrate 20 except for the opening patterns 33 and 36 and the light shifters 32 and 35. The pair of opening patterns 33 and 36 pass the light through and have a phase difference of 180.degree. relative to each other. The light shifters 32 and 35 surrounding the corresponding opening patterns 33 and 36 also have a phase difference of 180.degree. relative to each other. The light that passed through such a rim type phase inverting photomask is illustrated as a light intensity graph in FIG. 3B, wherein the neighboring light shifters 32 and 35 are offset from each other in phase, reducing the light intensity. When light having an intensity profile shown in FIG. 3B irradiates a photoresist film (not shown), an undesirable opening may be formed in the photoresist film.
FIG. 3C is a cross-sectional view taken along line IIIC--IIIC of a rim type phase inverting photomask. As described above, the phases of the opening pattern 33 and the phase shifter 32 are inverted relative to each other, the phases of the phase shifter 32 and the phase shifter 35 are inverted relative to each other, and the respective phases of the phase shifter 35 and the opening pattern 36 are inverted relative to each other. Such phase inversions depend on a thickness of the photomask substrate 20, so that the light shifter 32 and the opening pattern 33 have level differences in thickness, the light shifter 32 and the light shifter 35 have level differences in thickness, and the light shifter 35 and the opening pattern 36 have level differences with each other in thickness. In other words, the depth of the etch for the light shifter 35 is the same as the depth of the etch for the opening pattern 33.
The fabrication method of a conventional phase shifting photomask shown in FIG. 3C will now be described. First, as shown in FIG. 4A, a patterned layer 110 is formed on a patterned chromium layer 21, which is itself formed on a quartz substrate 20, in order to obtain a plurality of openings 111 formed through the layers 110 and the chromium layer 21.
As further shown in FIG. 4B, an undercut etching is carried out on the chromium layer 21 to form a plurality of chromium-eliminated regions 112, which will be employed as the phase shifters 32 and 35, as shown in FIG. 3A.
Referring to FIG. 4C, using the patterned layer 110 having the plurality of openings 111 as a mask, anisotropic etching is carried out to etch back the quartz substrate 20 to a depth sufficient to generate a phase shift. Such an etching process forms the opening patterns 33 and 36, as shown in FIG. 4C. Then, a patterned layer 115 having an opening 116 is formed, as shown in FIG. 4D. The quartz substrate 20 is etched through the opening 116 to a depth sufficient to generate a phase shift of 160.degree. to 200.degree., and preferably 180.degree.. As a result, as shown in FIG. 4E, the pair of opening patterns 33 and 36 and the pair of light shifters 32 and 35 are formed having a depth sufficient to invert the incoming light phase. This is accomplished using photolithographic exposure and etching techniques well known in the art.
However, such a conventional phase shifting photomask fabrication process leads to several disadvantages in carrying out the undercut etching. For example, increased viscosity between the photoresist film and the chromium layer 21 causes an etchant solution to infiltrate between the chromium layer 21 and the resist film 115, thereby disadvantageously etching the chromium layer 21.
Another conventional phase shifting photomask fabrication process that does not utilize an undercut etching process is shown in FIGS. 5A-5D. As shown in FIG. 5A, a chromium layer 21 is formed on the quartz substrate 20. A patterned layer 51 having a plurality of openings 52 is formed on the chromium layer 21. Using the patterned layer 51 as a mask, the chromium layer 21 and a certain amount of the quartz substrate 20 underneath the chromium layer 21 are etched. The patterned layer 51 is then removed, as shown in FIG. 5B. The etching into the quartz substrate 20 is carried out to a depth sufficient to generate a phase shift.
An further shown in FIG. 5C, another patterned layer 61 having a plurality of openings 62 is formed on the structure of FIG. 5B. The exposed opaque layer 21 is selectively etched, and the patterned layer 61 is removed, completing the conventional phase shifting photomask having an opening pattern 33 and a light shifter 35 as shown in FIG. 5D. Here, the patterned layer 61 should be accurately position-aligned so as to obtain a light shifter 35 of a desired dimension. Also, if the patterned layer 61 is slanted to one side, mask resolution may be degraded due to a difference in width between the light shifters.
Such a conventional phase shifting photomask fabrication method requires extremely accurate alignment, often leading to difficulties in the process of fabrication. Further, when etching the opaque layer 21 in order to form the opening pattern 33 and the light shifter 35, because of differences in etching regions, the quartz substrate 20 having an opening pattern may be partially etched due to a micro loading effect, and a photomask having a desired phase shift will not be produced.